Semiconductor integrated circuits typically include multiple levels of metallization to provide electrical connections between the large number of active semiconductor devices. Advanced integrated circuits, particularly those for microprocessors, may include five or more metallization levels. In the past, aluminum has been the favored metallization, but copper has been developed as a metallization for advanced integrated circuits.
A typical metallization level is illustrated in the cross-sectional view of FIG. 1. A lower-level layer 10 includes a conductive feature 12. If the lower-level layer 10 is a lower-level dielectric layer, such as silica or other insulating material, the conductive feature 12 may be a lower-level copper metallization, and the vertical portion of the upper-level metallization is referred to as a via since it interconnects two levels of metallization. If the lower-level layer 10 is a silicon layer, the conductive feature 12 may be a doped silicon region, and the vertical portion of the upper-level metallization is referred to as a contact because it electrically contacts silicon. An upper-level dielectric layer 14 is deposited over the lower-level dielectric layer 10 and the lower-level metallization 12. There are yet other shapes for the holes including lines and trenches. Also, in dual damascene and similar interconnect structures, as described below, the holes have a complex shape. In some applications, the hole may not extend through the dielectric layer. The following discussion will refer to only via holes, but in most circumstances the discussion applies equally well to other types of holes with only a few modifications well known in the art.
Conventionally, the dielectric is silicon oxide formed by plasma-enhanced chemical vapor deposition (PECVD) using tetraethylorthosilicate (TEOS) as the precursor. However, low-k materials of other compositions and deposition techniques are being considered, and the invention is equally applicable to them. Some of the low-k dielectrics being developed can be characterized as silicates, such as fluorinated silicate glasses. Hereafter, only silicate (oxide) dielectrics will be directly described, but the invention is applicable in large part to other dielectric compositions.
A via hole is etched into the upper-level dielectric layer 14 typically using, in the case of silicate dielectrics, a fluorine-based plasma etching process. In advanced integrated circuits, the via holes may have widths as low as 0.18 μm or even less. The thickness of the dielectric layer 14 is usually at least 0.7 μm, and sometimes twice this, so that the aspect ratio of the hole may be 4 or greater. Aspect ratios of 6 and greater are being proposed. Furthermore, in most circumstances, the via hole should have a vertical profile.
A liner layer 16 is conformally deposited onto the bottom and sides of the hole and above the dielectric layer 14. The liner 16 performs several functions. It acts as an adhesion layer between the dielectric and the metal since metal films tend to peel from oxides. It acts as a barrier against the inter-diffusion between the oxide-based dielectric and the metal. It may also act as a seed and nucleation layer to promote the uniform adhesion and growth and possibly low-temperature reflow for the deposition of metal filling the hole and to nucleate the even growth of a separate seed layer.
A metal layer 18, for example, of copper is then deposited over the liner layer 16 to fill the hole and to cover the top of the dielectric layer 14. Conventional aluminum metallizations are patterned into horizontal interconnects by selective etching of the planar portion of the metal layer 18. However, a preferred technique for copper metallization, called dual damascene, forms the hole in the dielectric layer 14 into two connected portions, the first being narrow vias through the bottom portion of the dielectric and the second being wider trenches in the surface portion which interconnect the vias. After the metal deposition, chemical mechanical polishing (CMP) is performed which removes the relatively soft copper exposed above the dielectric oxide but which stops on the harder oxide. As a result, multiple copper-filled trenches of the upper level, similar to the conductive feature 12 of the next lower level, are isolated from each other. The copper filling the trenches acts as horizontal interconnects between the copper-filled vias. The combination of dual damascene and CMP eliminates the need to etch copper. Several layer structures and etching sequences have been developed for dual damascene, and other metallization structures have similar fabrication requirements.
Filling via holes and similar high aspect-ratio structures, such as experienced in dual damascene, has presented a continuing challenge as their aspect ratios continue to increase. Aspect ratios of 4:1 are common, and the value will further increase. An aspect ratio is defined as the ratio of the depth of the hole to the narrowest width of the hole, usually near its top surface. Via widths of 0.18 μm are also common, and the value will further decrease. For advanced copper interconnects formed in oxide dielectrics, the formation of the barrier layer tends to be distinctly separate from the nucleation and seed layer. The diffusion barrier may be formed from a bilayer of Ta/TaN, W/WN, or Ti/TiN, or of other structures. Barrier thicknesses of 10 to 50 nm are typical. For copper interconnects, it has been found necessary to deposit one or more copper layers to fulfil the nucleation and seed functions. The following discussion will address the formation of the copper nucleation and seed layer as well as the final copper hole filling.
The deposition of the metallization by conventional physical vapor deposition (PVD), also called sputtering, is relatively fast. A DC magnetron sputtering reactor has a target composed of the metal to be sputter deposited and which is powered by a DC electrical source. The magnetron is scanned about the back of the target and projects its magnetic field into the portion of the reactor adjacent the target to increase the plasma density there to thereby increase the sputtering rate. However, conventional DC sputtering (which will be referred to as PVD in distinction to other types of sputtering to be introduced) predominantly sputters neutral atoms. The typical ion densities in PVD are less than 109 cm−3. PVD also sputters atoms into a wide angular distribution, typically having a cosine dependence about the target normal. Such a wide distribution is disadvantageous for filling a deep and narrow via hole 22 illustrated in FIG. 2, in which a barrier layer 24 has already been deposited. The large number of off-angle sputter particles cause a copper layer 26 to preferentially deposit around the upper corners of the hole 22 and form overhangs 28. Large overhangs further restrict entry into the hole 22 and at a minimum cause inadequate coverage of the sidewalls 30 and bottom 32 of the hole 22. At worst, the overhangs 28 bridge the hole 22 before it is filled and create a void 34 in the metallization within the hole 22. Once a void 34 has formed, it is almost impossible to reflow it out by heating the metallization to near its melting point. Even a small void introduces serious reliability problems. If a second copper deposition step is planned, such as by electroplating, the bridged overhangs make it impossible.
One approach to ameliorate the overhang problem is long-throw sputtering in a conventional reactor. In long-throw sputtering the target is spaced relatively far from the wafer being sputter coated. For example, the target-to-wafer spacing is at least 50% of wafer diameter, preferably is more than 90%, and more preferably is more than 140%. As a result, the off-angle portion of the sputtering distribution is preferentially directed to the chamber walls, but the central-angle portion remains directed to the wafer. The truncated angular distribution causes a higher fraction of the sputter particles to be directed deeply into the hole 22 and reduces the extent of the overhangs 28. A similar effect is accomplished by positioning a collimator between the target and wafer. Because the collimator has a large number of holes of high aspect ratio, the off-angle sputter particles strike the sidewalls of the collimator, and only the central-angle particles are passed. Both long-throw targets and collimators disadvantageously reduce the flux of sputter particles reaching the wafer. That is, they reduce the sputter deposition rate. The reduction becomes more pronounced as longer throws and stricter collimation become required for via holes of increasing aspect ratios. Long throw is further limited by the the longer substrate-to-target distance over which the sputtered particles must travel. At the few milliTorr of argon pressure used in conventional PVD even with long throw, there is a greater possibility of the argon scattering the sputtered particles. Hence, the geometric selection of the forward particles is decreased. A yet further problem with both long throw and collimation is that the reduced copper flux necessitates a longer deposition period. This not only reduces throughput, it also tends to increase the maximum temperature the wafer experiences during sputtering. Long throw reduces overhangs and provides good coverage in the middle and upper portions of the sidewalls, but the lower sidewall and bottom coverage are inferior.
Another technique for deep hole filling is sputtering using a high-density plasma (HDP) in a sputtering process called ionized metal plating (IMP). A high-density plasma is defined as one having an average plasma density across the plasma, exclusive of the plasma sheaths, of at least 1011 cm−3, and preferably at least 1012 cm−3. In IMP deposition, a separate plasma source region is formed in a region away from the wafer, for example, by inductively coupling RF power into the chamber through an electrical coil wrapped around a plasma source region between the target and the wafer. This configuration is commercially available from Applied Materials of Santa Clara, Calif. as the HDP PVD Reactor. Other HDP sputter reactors are available. The higher power ionizes not only the argon working gas, but also significantly increases the ionization fraction of the sputtered atoms, that is, produces metal ions. The wafer either self-charges to a negative potential or is RF biased to control its DC potential. The metal ions are accelerated across the plasma sheath as they approach the negatively biased wafer. As a result, their angular distribution becomes strongly peaked in the forward direction so that they are drawn deeply into the via hole. Overhangs become much less of a problem in IMP sputtering, and bottom coverage and bottom sidewall coverage are relatively high.
IMP deposited metals, however, suffer many problems. First, HDP sputter reactors are expensive. Secondly, IMP sputtering using a remote plasma source is usually performed at a higher pressure of at least 30 milliTorr. The higher pressures and a high-density plasma produce a very large number of argon ions, which are also accelerated across the plasma sheath to the surface being sputter deposited. The high-energy argon ions cause a number of problems. The argon ion energy is dissipated as heat directly into the film being formed. Copper will dewet from tantalum nitride and other barrier materials at the elevated temperatures experienced in IMP, even at temperatures as low at 50 to 75° C. Further, the argon tends to become embedded in the developing film, which cannot be a good effect. Experimentally, it is observed that IMP deposits a copper film 36, as illustrated in the cross-sectional view of FIG. 3, having a surface morphology that is very rough or even discontinuous. Such a film does not promote hole filling, particularly when the liner is being used as the electrode for electroplating.
Another technique for depositing copper is sustained self-sputtering (SSS), as is described by Fu et al. in U.S. patent application Ser. No. 08/854,008, filed May 8, 1997 and by Fu in U.S. Ser. No. 09/373,097, filed Aug. 12, 1999, now issued as U.S. Pat. No. 6,183,614. At a sufficiently high plasma density adjacent a copper target, a sufficiently high density of copper ions develops that the copper ions will resputter the copper target with yield over unity. The supply of argon working gas can then be eliminated or at least reduced to a very low pressure while the copper plasma persists. Aluminum is not subject to SSS. Some other materials, such as Pd, Pt, Ag, and Au can also undergo SSS.
Depositing copper by sustained self-sputtering of copper has a number of advantages. The sputtering rate in SSS tends to be high. There is a high fraction of copper ions which can be accelerated across the plasma sheath and toward a biased wafer, thus increasing the directionality of the sputter flux. Chamber pressures may be made very low, often limited by leakage of backside cooling gas, thereby reducing wafer heating from the argon ions and decreasing scattering of the metal particles by the argon. It has, however, been found that standard long-throw PVD chambers will not support SSS of copper.
Techniques and reactor structures have been developed to promote sustained self-sputtering. It has been observed that some sputter materials not subject to SSS because of sub-unity resputter yields nonetheless benefit from these same techniques and structures, presumably because of partial self-sputtering, which results in a partial self-ionized plasma (SIP). Furthermore, it is often advantageous to sputter copper with a low but finite argon pressure even though SSS without any argon working gas is achievable. Hence, SIP sputtering of copper is the preferred terminology for the more generic sputtering process involving a reduced or zero pressure of working gas so that SSS is a type of SIP.
Copper may also be deposited by chemical vapor deposition (CVD) using metallo-organic precursors, such as Cu-HFAC-VTMS, commercially available from Schumacher in a proprietary blend with additional additives under the trade name CupraSelect. A thermal CVD process may be used with this precursor, as is very well known in the art, but plasma enhanced CVD (PECVD) is also possible. The CVD process is capable of depositing a nearly conformal film even in the high aspect-ratio holes being considered here. The original concept was to CVD deposit a copper film as a thin seed layer, and then use PVD or other technique for final copper hole filling. The proposed concept was based on the expense associated with CVD processes and equipment needed for filling a relatively wide via hole of perhaps 0.25 to 0.5 μm width. However, CVD copper seed layers have been observed to be almost invariably rough. The roughness detracts from its use as a seed layer and more particularly as a reflow layer promoting the low temperature reflow of after deposited copper deep into hole. Also, the roughness indicates that a relatively thick CVD copper layer of the order of 50 nm needs to be deposited to reliably coat a continuous seed layer. For the narrower via holes now being considered, a CVD copper seed layer of the necessary thickness may nearly fill the hole anyway. However, complete fills performed by CVD tend to suffer from center seams, which may impact device reliability.
Another, combination technique uses IMP sputtering to deposit a thin copper nucleation layer, sometimes referred to as a flash deposition, and a thicker CVD copper seed layer is deposited on the IMP layer. However, as was illustrated in FIG. 3, the IMP layer 36 tends to be rough, and the CVD layer tends to conformally follow the roughened substrate. Hence, the CVD layer over an IMP layer will also be rough.
Electrochemical plating (ECP) is yet another copper deposition technique that is being developed and is likely to become the preferred commercial filling process. In this method, the wafer is immersed in a copper electrolytic bath. The wafer is electrically biased with respect to the bath, and copper electrochemically deposits on the wafer in a generally conformal process. Electroless plating techniques are also available. Electroplating and its related processes are advantageous because they can be performed with simple equipment at atmospheric pressure, the deposition rates are high, and the liquid processing is consistent with the subsequent chemical mechanical polishing.
Electroplating, however, imposes its own requirements. A copper seed and adhesion layer is required on top of the barrier layer, such as of Ta/TaN, to nucleate the electroplated copper and adhere it to the barrier material. Furthermore, the generally insulating structure surrounding the via hole 22 requires that an electroplating electrode be formed between the dielectric layer 14 and the via hole 22. Tantalum and other barrier materials are typically relatively poor electrical conductors, and the usual nitride sublayer of the barrier layer 24 which faces the via hole 22 (containing the copper electrolyte) is even less conductive for the long transverse current paths needed in electroplating. Hence, a good conductive seed and adhesion layer must be deposited if the electroplating is to effectively fill the bottom of the via hole.
A copper seed layer deposited over the barrier layer 24 is typically used as the electroplating electrode. However, its integrity must be assured, and a continuous, smooth, and uniform film is preferred. Otherwise, the electroplating current will be directed only to the areas covered with copper or be preferentially directed to areas covered with thicker copper. Depositing the copper seed layer presents its own difficulties. An IMP deposited seed layer provides good bottom coverage in high aspect-ratio holes, but its sidewall coverage is so small that the resulting thing films can be rough to the point of discontinuity, leading to sidewall voiding. A thin CVD deposited seed is also too rough. A thicker CVD seed layer or CVD copper over IMP copper may require an excessively thick seed layer to achieve the required continuity. Also, the electroplating electrode primarily operates on the entire hole sidewalls so that high sidewall coverage is desired. Long throw provides a adequate sidewall coverage, but the bottom coverage is not sufficient.
Accordingly, a better method is desired for filling a via hole with copper.
Sputtering of copper has been observed to create problems not observed with sputtering of other electronic materials. Scattered copper atoms have been observed to diffuse much further into narrow recesses in the sputtering apparatus, especially for low-pressure SSS and SIP. Since sputtering, particularly at the high power levels associated with SSS and SIP, involves high voltages, dielectric isolators must separate the differently biased parts. These isolators tend, however, to become coated with copper during SSS sputtering and thus require frequent replacement or cleaning.
As a result, it is desired to protect such isolators from copper deposition.
A standard part of a sputtering chamber is a chamber shield which protects the chamber walls from deposition. The shield rather than the wall is coated and is much more easily removed or cleaned when the sputter coating builds up to excessive thickness. Typically, the shield is metallic and is electrically grounded to act as the grounding plane for the cathode target. However, it has become known that an auxiliary electrically floating shield placed around the upper part of the chamber near the target allows for sputtering at lower pressure. Such a floating shield has typically been simply supported on a shield isolator without any clamping. However, an unclamped shield introduces additional mechanical motion during the thermal cycling, which impacts the tolerances required of plasma dark spaces and proper target biasing. Shield flexing may cause excessive particulate flaking. Improper gaps and configurations between the shields and isolators cause electrical shorts. If the unclamped shield has moved during thermal cycling, there often is no assurance that it returns to its original position upon cooling so that on the next cycle it may be even further askew.
For these reasons, better alignment is desired for unclamped shields.
Plasma sputtering at low pressures, particularly those associated with SIP and SSS, may introduce a problem with igniting the plasma. Ignition of the plasma involves a different set of conditions than does its continued excitation. Often, a higher-pressure working gas, such as argon, needs to be admitted to the chamber to produce ignition. However, the film sputter coated in the presence of a larger amount of argon is not likely to conform to the type of film desired in low-pressure sputtering. Furthermore, igniting the plasma can be problematical resulting in long and undependable ignition sequences.
Accordingly, it is desired to provide better ignition of plasma for low-pressure sputtering, particularly of copper.